Transistors employing an organic semiconductor are being developed actively in competition with silicon transistors based on crystalline silicon technology. The organic semiconductor has features of an organic material such as light-weight, flexibility, variety, and durability, and further has advantages that it can be formed by a low temperature process of about 100° C. and can be produced by a liquid process such as printing and spin coating. Therefore, the organic transistor can be formed on a plastic substrate, or in a larger display screen, which has not been achieved by crystal silicon semiconductors. Therefore the organic transistor is promising in application to novel devices such as flexible electronic paper sheets, and information tags.
A usual organic semiconductor has a carrier mobility on a level of 10−4 to 10−2 cm2/Vs, which is lower by one or more decimal digits than in silicon semiconductors. Owing to this high resistance, a large current is not readily obtainable and the operation frequency is lower, disadvantageously. For obtaining higher mobility, it is effective to arrange regularly the organic semiconductor layers to enlarge the overlap of the conjugation planes as large as possible. A simple method for arranging a liquid crystal substance for a display element is a rubbing method. In Patent Literature 1 (shown later), a fluorine type amorphous polymer is laminated to an insulating oxide film, and thereon an organic semiconductor is arranged by rubbing treatment to achieve a mobility of a level of 10−3 cm2/Vs.
On the other hand, to solve the problem of contamination of impurity caused by the cloth and nonuniformity in the rubbing method, optical orientation is disclosed in which the film is made anisotropic by irradiation with light for orientation. However, the optical orientation is applicable to limited kinds of liquid crystal materials as described in Patent Literature 3.
Generally, in the field effect transistor, the drain current in the saturation area can be derived according to Equation (I) below.Id=μ(W/2L)Ci(Vg−Vth)2  (I)where Id is a drain current (A), μ is a mobility (cm2/Vs), W is a channel breadth (cm), L is a channel length, Ci is a capacity (F/cm2) of a gate insulating layer, Vg is a gate voltage (V), Vth is a threshold voltage of a transistor. The value of Vth is obtained by extrapolation in the relation of the square root of the drain current and the gate voltage to the drain current Id=0.
In use of the transistor as a switching device, the ratio of the current flowing between a source electrode and a drain electrode in a turned-on state to that in a turned-off state (on-off ratio) should be not less than 104, preferably is not less than 106. However, in organic semiconductors, the ion current is small owing to the low mobility as mentioned above and the off-current is large owing contamination in the organic semiconductors. Therefore, sufficiently high on-off ratio is not obtainable with the organic semiconductors. The field effect type organic transistor employing an organic semiconductor does not satisfy the necessary practical characteristics at the moment.    Patent Literature 1 Japanese Patent Application Laid-Open No. H07-221367    Patent Literature 2 Japanese Patent Application Laid-Open No. H10-182821    Patent Literature 3 Japanese Patent Application Laid-Open No. 2001-40209